1. Field of the Invention
This invention relates to an apparatus and process for drying of carbonaceous feedstocks by electrical resistance heating. It particularly relates to an improved apparatus for conducting an improved electrical resistance heating process in which the energy cost of drying is reduced, and the process is more efficient and faster and can accept carbonaceous feedstocks with higher moisture contents, larger ranges of particle size, and more variable particle size distributions as compared with previously described electrical resistance processes for the drying of carbon.
2. Description of the Related Art
The drying of wet powdered or granular solids requires the vaporization of the moisture followed by its physical removal, usually by a stream of air or other gas. The process is hastened by the application of heat either to the wet material or to the gas stream that is made to flow over or through the material. When the powdered or granular material is carbon, electrical resistance heating may be applied directly to the solid, as is well known in the prior art relating to the activation of various carbonaceous feedstocks or to the reactivation of spent activated carbons, as described, for example, in U.S. Pat. No. 5,089,457 and prior patents cited therein. Effective rapid drying of wet carbons requires temperatures only about or modestly above the boiling point of water, such as temperatures in the range from 100.degree. C. to about 150.degree. C., whereas the activation of carbon feedstocks or the reactivation of spent activated carbons commonly requires temperatures of about 750.degree. C. or higher.
When a moist or wet carbonaceous feedstock is activated or a wet spent activated carbon is reactivated by electrical resistance heating, the carbon is either dried prior to the activation or reactivation step, or else some or substantially all of the moisture is removed in the process of the activation or reactivation. In the latter case, as the vaporized moisture moves toward the furnace exit, through and past the section that preheats the carbon, it recondenses and offers increased resistance to the movement of other gases (such as CO and H.sub.2) that are produced in the activation process or that are desorbed in the reactivation of spent activated carbons and that must escape to the exit. Still another disadvantage is the uneven current distribution that results from the variation in electrical resistance in different portions of the wet carbon.
It is accordingly an object of this invention to provide an improved apparatus for the drying of wet granular or powdered carbon by electrical resistance heating.
It is another object of this invention to provide such improvement when the carbon contains any amount of moisture up to and including full saturation, so long as no free liquid water is present.
It is another object of this invention to provide such improvement when the carbon is a carbonaceous feedstock, such as wood char, coconut char, or petroleum coke, for the production of activated carbon, or is an exhausted activated carbon that is to be reactivated.
It is another object of this invention to provide that the electric current passing through the carbon being dried is distributed uniformly therethrough.
It is yet another object of this invention to insure that the moisture being removed from the carbon does not recondense on the carbon before it escapes.
The achievements of these and other objects will be apparent from the following description of the subject invention.